There are essentially two different types of trailers or semi-trailers of sheet and post side panel construction in use in the trailer industry today. The structural differences between the two are well known to be that, (a) the exterior skin side panel construction consists of side sheets on the exterior surface of the panel fastened to vertical posts or stiffeners positioned on the interior surface and in contrast, (b) the exterior post side panel construction consists of side sheets on the interior surface of the panel fastened to vertical posts or stiffeners positioned on the exterior surface.
These two different types of side wall constructions used in trailers and semi-trailers uniquely satisfy different needs of the trailer user. The exterior post configuration essentially provides a smooth interior surface without the need for interior lining which results in greater inside width and cube. Where inside width is not a critical factor the exterior skin configuration provides a relatively smooth exterior surface which aids in the installation of signs and decals, results in less wind resistance encountered over-the-road and, is less susceptible to damage from tree limbs, other obstacles, etc.
The wide use of both of these types of trailers requires that trailer manufacturers inventory the necessary components to build both types of trailers. The geometrical differences of both side panel constructions require modifications in the upper and lower longitudinal support members in order to provide maximum interior dimensions within the maximum exterior dimensions allowed by state laws. This requirement most often results in two totally unique top rail assemblies.
In manufacturing top rails for trailers having open tops, the trailer manufacturing industry often utilizes roll formed sections, but with increased loads handled by the transportation industry requiring stronger structural elements, the inherent limitations of roll-forming to form these elements from thicker gauge metals are quite apparent.
To decrease the weight of individual elements and offset the required greater thickness, a multiple piece top rail is being used extensively. Often such top rail assemblies were believed to require three or more pieces even though a two piece assembly is more desirable.
The use of two piece top rails while having substantial advantages, also raised new problems. Principally, the problems arose from the fact that to form two piece top rails and use them for both exterior post and exterior skin side panel configurations two pieces for each configuration or a total of four pieces were actually required. These four individual pieces, each possessing its outer unique cross section prevent interchangeability between top rail assemblies for exterior post and exterior skin constructions.
In roll forming these structural elements, a separate set of rolls is required for each different cross section. Therefore, as many as four different sets of rolling dies would be required to satisfy design requirements. The cost of a set of rolls to form one piece of a top rail assembly is quite substantial and therefore there is a distinct advantage to limit the number of roll forming dies required. Previous attempts by designers to cope with this costly problem have still resulted in a minimum of three uniquely different cross sections and a substantial cost increase.
Another drawback to the four pieces required to meet the demands of both an exterior post and exterior skin construction is that double inventories must be maintained by the trailer manufacturer prior to the trailer construction.
An important consideration in any acceptable design for a two piece top rail is the requirement that upon installation in either an exterior skin or exterior post construction certain dimensional characteristics must remain constant. Among these are roof bow length and height or arc, overall exterior width, inside clear width, roof tarp protection and side panel height in relation to the overall height of the vehicle. To achieve these characteristics interchangeability must not be sacrificed.
In order to meet the requirements of maintaining constant roof bow length for trailer constructions of an exterior skin or exterior post, it is important that the top rail assemblies make due allowance for the fact that the vertical posts are not in the same transverse position in the trailer bed. For instance, for the exterior skin side panel construction, corrugations on the exterior skin are usually required necessitating that the posts be moved transversely inwardly to permit these corrugations on the exterior skin to be within the maximum legal allowable exterior width. For the exterior posts construction the posts are positioned laterally outwardly to the maximum degree permissable under the law governing exterior width of trailers and since no panel is to be positioned transversely outwardly of the posts, the posts are positioned further outwardly than in the exterior skin panel construction. Because the posts support the top rail assembly, the variation in spacing between transversely opposed posts must be considered in any acceptable design of the top rail assembly usable for either of the exterior post or exterior skin panel construction.